The present invention relates to a sound-dampened driving apparatus for fasteners wherein a main valve means is arranged above a working cylinder of the apparatus and movable within a cylindrical bore. When the main valve means is in its lower at rest position, the main valve means separates the working cylinder from a source of compressed air and connects the cylinder to the atmosphere. When the main valve means is in its upper actuating position, the working cylinder is connected to the source of compressed air and the valve means blocks the cylinder connection to the atmosphere. The space above the main valve member within the cylindrical bore is capable of being alternately connected to either the atmosphere or compressed air, and includes a sound dampening means arranged in the space above the main valve member.
Compressed air-operated driving apparatuses for fasteners serve to process fasteners, such as staples, nails, pins and screws, and drive the same into a workpiece. A complete working cycle of such driving apparatuses is composed of a driving and a blow-out phase. During the driving phase, a piston is driven by compressed air, after the opening of the auxiliary controlled main valve, and is highly accelerated. A fastener is positioned in the path of the piston stroke and driving into the workpiece by the accelerated piston in the driving phase. The residual excess energy of the piston which is not used during the driving operation or phase, is generally intercepted at the end of the stroke by a buffer element. During the blow-out phase, the working piston returns to its initial starting position thereby forcing the air present in the working cylinder into the atmosphere.
Both working cycles of these driving apparatuses generate pulse-like airborne sounds and sounds conducted through the housing solids of different intensity. The intensity level of airborne sound which is radiated in the driving phase is created by the piston striking against the buffer element at the end of the stroke. In the blow-out phase, the radio-frequency noises are caused by the outflowing compressed air from the apparatus.
It has been suggested to reduce the radiation of noises from such devices during the blow-out phase by suitable sound absorbers. For example, it is known to use as a sound absorber a thin perforated plastic ring (German disclosure letter No. 24 22 222) or a tightly pressed wire cloth (German disclosure letter No. 27 24 220). It has been suggested also to discharge the outflowing compressed air via a two-chamber system (German disclosure letter No. 28 27 279. Also, in view of the noise emission, it has been suggested to intercept the piston in the driving phase in such a manner that the braking step is timely extended. With one known driving apparatus, this is obtained with the aid of an air cushioning arrangement (German disclosure letter No. 25 04 094). The construction necessary for this, however, results in bulky driving apparatuses which are costly and prone to trouble.
It has also been suggested, in connection with a driving apparatus, to attach a ring of polyurethane foam material to the underside of the driving piston, with the ring striking against a buffer element of crosslinked polyurethane of a predetermined Shore hardness positioned in the cylinder at the end of the stroke. However, examinations have shown that an effective dissipation of the percussion energies and, accordingly, an optimum reduction of the airborne sound oscillations and the oscillations of sound conducted through solids can be obtained only by stretching the energy/time pattern during this braking step. In the case of this known arrangement, this is obtained only because of an extended braking path and increased mass of body. Because the mass of the body connected to the piston and the brake path provide for the dissipation of the energy of percussion are too small, a soft dampening by the foam material dampening element will not occur. Accordingly, the dampening will be taken over almost exclusively by the hard buffer element on a very short brake path (German publication letter No. 23 39 163), a result which is insatisfactory.
It has also been suggested to design a buffer element in such a manner that in cooperation with the driving piston, the buffer element will experience a radial stretching (German disclosure letter No. 25 10 858). This known arrangement, too, suffers from several drawbacks. Owing to the central accumulation of the mass of the buffer element, a radial stretching takes place only due to a very high energy of percussion, such as occurs in connection with free shots. The residual energies which are not consumed in the driving operation or phase, do not effect a radial deformation of the buffer element, so that a shock-like and relatively hard dampening will result from normal operation. Also, the deformation of the buffer elements results in a considerable development of heat through internal frictional heat during the performance of the work of deformation. Accordingly, the working life of such buffer elements are negatively influenced and such devices are commercially unacceptable.
Another source of the development of noise during the driving phase lies in the movement of the valve member of the main valve. This is true, especially in connection with apparatuses having a high driving capacity. The resultant change in acceleration of the main valve member leads to shock-like braking operations which are effective to cause the development of a considerable noise. In addition, this process results in a considerable rebounding effect of the apparatus handle which the operator effectively feels. To overcome these problems, it has been suggested to position at the inner surface of the cap of the cylinder housing a dampening ring of soft material against which the main valve member strikes when being adjusted into the opening direction. Although such structures are intended to displace the bouncing noise in a direction towards the low frequency range, however, decisive noise dampening effects are not obtained, and such structures do not provide satisfactory dampening of the rebounding shock. Additionally, it has been suggested to compensate the rebound caused by the main valve member by a housing cap having a high specific gravity. This, however, results in a displacement of the center of gravity, and the apparatuses become top-heavy. By this measure, thus the operational comfort is rendered worse. Especially, in the case of heavy apparatuses, with the weight unbalanced, the wrist of the operator is considerably stressed.